Burn injury alters intestinal glutamine transport

Several studies have established that intestinal glutamine (GLN) metabolism is altered during catabolic states. It remains unclear whether these alterations are due to a defect in metabolism or in transport of the amino acid. The present study examines the kinetics of GLN transport across basolateral membrane vesicles (BLMV) of enterocytes obtained from control rats and rats subjected to 20% full-thickness scald burn, 48 hr previously. BLMV were prepared from freshly isolated enterocytes using differential centrifugation with separation on a Percoll density gradient. BLMV were enriched (10- to 12-fold) with Na⁺K⁺ATPase while markers for brush border membranes were impoverished. Previous studies from our laboratory indicated that, in this preparation, GLN transport is into an osmotically sensitive space, dependent on GLN concentration, linear up to 30 sec, and both temperature and Na⁺ dependent. Our results indicate that in thermal injury, initial rates of GLN uptake were depressed (y = 3.67 + 0.435X for burned rats vs y = 18.7 + 0.907X for controls, P < 0.01). Kinetic analysis of GLN uptake showed a marked decrease in transport V_max (81.8 ± 15 nmole/mg protein/15 sec for burned rats vs 185 ± 17 nmole/mg protein/15 sec for controls, P < 0.001). Transport K_m also decreased from 0.25 ± 0.004 mM for controls to 0.08 ± 0.03 mM glutamine for burned rats (P < 0.001). Kinetic studies performed at GLN levels >0.6 mM showed that GLN uptake proceeded by a nonsaturable process in both the control and burned rats. No significant alteration in this nonsaturable component was observed. The present results suggest that burn injury decreases the electrogenic, Na⁺-dependent, carrier-mediated transport of GLN across the basolateral membrane. At higher GLN concentrations (>0.6 mM), passive transport by the nonsaturable pathway occurs and this process is not altered during burn injury.